Delibird was delivering two Poké Balls to Jessie and James two Poké Balls. When they learned the Poké Balls contained a Charizard and an Aggron, they decided to use them to take over Pewter Gym and steal the Poké Balls in the Gym.
Using their disguises, they challenged Brock and Ash to a Double Battle. Team Rocket sent out their newly acquired Pokémon, while Brock and Ash sent out their Steelix and Donphan, respectively. Unfortunately, Team Rocket got an early advantage and appeared to be winning. The victory was short-lived when Max discovered Meowth stealing all of the Gym's Poké Balls. It was then revealed that the two were indeed Team Rocket. With the help of Forrest, they were sent blasting off.
At the end of the day, Delibird returned to take the Poké Balls back, as the Charizard and Aggron were originally intended for Cassidy and Butch, respectively. Before Team Rocket could plead with Delibird to return them, it had flown away. It is assumed Charizard was delivered to Cassidy successfully.